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A/C pipe size

I'm new to all this HVAC stuff so forgive me if I get my terminology wrong. We are currently shopping for a new A/C and furnace and were told by two contractors that the pipe coming from the outdoor A/C unit is 7/8" while our interior pipes are only 3/4". They recommended changing the tubing inside our house so that they are all 7/8".

One other contractor said that this wasn't necessary since it was a short run (probably about 25 feet or so) - he said if it was his house he wouldn't bother with changing it out and damaging the ceiling, etc.

Any thoughts on whether or not this change is necessary? Our goal is to have a good system that we don't need to think about for a long time and so I obviously want to make sure it's installed correctly (nor do I want to void any warranties by it NOT being installed correctly).

A/C line sizes are spelled out by the manufacturer by lenghts, type of refrigerant and size of unit needed. Your contractors should have access to this info. It is not a guessing game. If possible change the lines. If not then have them check with the manufacturer on weather the current lines will work.

The proposed size (tonnage) of the new unit would be useful to know. Most units running 3 tons or less capacity work well with a 3/4" lineset, above that manufacturers usually specify 7/8". Several require 1 1/8" for the 5 ton applications.

So that's the tricky 'in between' capacity where pro opinion's may vary. Sounds like original installer opted for 3/4" - perhaps due to short length. Advice to upsize is sound, but likely adds $$ to job cost. Impact on final performance would be pretty small, but in my experience it always pays to do it right the first time.. I'd say replace it.